The present invention relates to a method and apparatus for the formation of objects from a flat metallic sheet within a ram press and, more particularly, to such a method and apparatus for the manufacture of shells used to close the ends of metal cans.
One common way of packaging liquids, particularly beverages such as beer, soft drinks, juices and the like, is within cans typically formed from metal stock. In such cans, the can body is often manufactured to include the can side walls, and may include an attached bottom end. The upper end, which includes the means by which the can is opened, is manufactured separately and attached to the can body after the can has been filled.
Due to the carbonated nature of many of the beverages contained within such cans, it is necessary for the upper can end, often referred to within the art as a shell, to be able to withstand the pressures present within the can. Accordingly, typical shells are designed with a flat panel surface surrounded by a countersunk groove from which an almost vertical chuckwall rises. A curled lip portion extends outwardly from the upper end of the chuckwall, with the lip portion having a hook-like cross-section. Once the can body has been filed, the shell is placed atop the can with the lip portion cooperating with a hook-like projection at the uppermost edge of the can side wall. The shell lip portion and can hook portion are then seamed together in mutual engagement, sealing the can closed.
In view of the large quantities of cans and ends that are manufactured, it is economically very desirable to form the can shells from as thin a stock material as possible while retaining the necessary pressure-resistant strength therein.
Typically, shells are manufactured by formation within a ram press. This method of formation has in the past resulted in limitations upon the thinness of material used for shells. The relative sharp radius of the curves imparted to the shell material to form the countersink results in significant thinning of the material as these curves are formed. This weakens the shell at the very locations where maximum strength is required. Moreover, this can result in splitting of the shell material during formation, after which the shell must be discarded. Thus, the shell must be formed from stock material of an initial thickness greater than the overall thickness required for proper shell strength.
One method through which it has been sought to overcome this problem is to manufacture the shell and then subsequently reform the shell in a conversion press. Such a method is disadvantageous, however, in that it requires significant investment in additional equipment and a substantial increase in the time and energy required for shell manufacture. To further compound these drawbacks, the curled lip for seaming the can end to the can body must be formed in yet a third machine, typically by rolling the shell edge prior to the reforming operation.
A second approach is to provide a double action press which can perform the initial manufacture and subsequent reforming within a single machine. While such a method would decrease the time needed to manufacture a shell, the specialized equipment represents a significant financial burden in replacing presses presently in service. Moreover, curling must still be performed in separate equipment.
What is needed, therefore, is a method and apparatus for the manufacture of shells that will permit the use of thinner stock material while maintaining or increasing the strength within the completed shell. Such a method and apparatus should be compatible with conventional ram presses currently in use, and should be capable of producing a fully completed shell.